System and method for controlling a charge pump

ABSTRACT

A charge pump controller for controlling a charge pump adapted to convert an input voltage into an output voltage with a conversion ratio is presented. The charge pump is operable in a plurality of modes corresponding to different conversion ratios. The controller includes a first selector for selecting a mode of operation of the charge pump. The first selector comprises a first input for coupling to a voltage supply; and a second input for coupling to a source signal. The first selector identifies a target value of the output voltage. The selector calculates a product of the conversion ratio and the input voltage. The selector compares the product with the target value and selects a mode of operation of the charge pump by increasing or decreasing the conversion ratio based on the comparison. The selector maintains the conversion ratio for a length of time before decreasing the conversion ratio.

This is a Divisional application of U.S. patent application Ser. No.16/135,687, filed on Sep. 19, 2018 which is herein incorporated byreference in its entirety, and assigned to a common assignee.

TECHNICAL FIELD

The present disclosure relates to a charge pump controller and a methodof operating a charge pump. In particular, the present disclosurerelates to charge pump controller for use with an audio amplifier.

BACKGROUND

Audio amplifiers, such as class-D amplifiers may be used in varioussignal amplification circuits. The audio amplifier requires an inputvoltage that can be above the voltage of the battery powering the signalamplification circuit. For this reason, a boost circuit is provided forincreasing the battery voltage above a certain reference voltage. Thereference voltage may be constant or may vary dynamically to track theaudio signal.

The boost circuit can be implemented as an inductive or a capacitiveconverter. Capacitive converters such as capacitive multipliers have asmaller size than inductive converters and for this reason are oftenpreferred. However, current systems based on capacitive multipliers aresensitive to variations in the battery voltage. For instance, if theaudio signal increases, the audio amplifier may require more power anddeplete the battery. Such changes in battery voltage lead to significantvariations in the voltage generated by the capacitive converter, whichin turn reduces the quality of the audio signal.

SUMMARY

It is an object of the disclosure to address one or more of theabove-mentioned limitations. According to a first aspect of thedisclosure there is provided a method of controlling a charge pump forconverting an input voltage into an output voltage with a conversionratio, the charge pump being operable in a plurality of modescorresponding to different conversion ratios, the method comprising thesteps of identifying a target value of the output voltage; calculating aproduct of the conversion ratio and the input voltage; comparing theproduct with the target value; and selecting a mode of operation of thecharge pump by increasing or decreasing the conversion ratio based onthe comparison; and maintaining the conversion ratio associated with theselected mode of operation for a length of time before decreasing theconversion ratio.

For instance, the length of time may be a pre-set value or aprogrammable value.

Optionally, the conversion ratio may be an integer.

Optionally, the method comprises identifying a plurality of products fordifferent values of the conversion ratio, and selecting a conversionratio associated with a product that is the closest to the target value.The product closest to the target value may be less than the targetvalue, equal to the target value or greater than the target value.

Optionally, the conversion ratio corresponds to a product that is equalor greater than the target value.

Optionally, the target value may vary with time and identifying thetarget value comprises sensing a source signal to estimate the targetvalue.

For example, the target value may be proportional to the source signal.The source signal may be an audio signal such a digital audio signal.

Optionally, the method comprises recording amplitude maxima of thesource signal.

Optionally, the method comprises calculating a ratio of the product overthe target value to select the mode of operation.

Optionally, the method further comprises detecting a rate of change ofan input signal function of the input voltage; and upon identifying thata magnitude of the rate of change has crossed a threshold value,generating a reference signal, comparing the output voltage with thereference signal; and selecting a mode of operation of the charge pumpbased on the comparison.

Optionally, the reference signal is varying with a rate of change thatis less than a rate of change of the input signal.

According to a second aspect of the disclosure, there is provided acharge pump controller for controlling a charge pump adapted to convertan input voltage into an output voltage with a conversion ratio, thecharge pump being operable in a plurality of modes corresponding todifferent conversion ratios, the controller comprising a first selectorfor selecting a mode of operation of the charge pump; the first selectorcomprising a first input for coupling to a voltage supply proving theinput voltage; and a second input for coupling to a source signal; thefirst selector being configured to identify a target value of the outputvoltage; calculate a product of the conversion ratio and the inputvoltage; compare the product with the target value; and select a mode ofoperation of the charge pump by increasing or decreasing the conversionratio based on the comparison; and maintaining the conversion ratioassociated with the selected mode of operation for a length of timebefore decreasing the conversion ratio.

Optionally, the conversion ratio may be an integer.

Optionally, the first selector is configured to identify a plurality ofproducts for different values of the conversion ratio, and to select aconversion ratio associated with a product that is the closest to thetarget value.

Optionally, the first selector is configured to calculate the targetvalue based on the source signal.

Optionally, the first selector is configured to record amplitude maximaof the source signal.

Optionally, the charge pump controller as claimed further comprises aslew rate detector adapted to detect a rate of change of an input signalfunction of the input voltage and to generate a reference signal uponidentifying that a magnitude of the rate of change has crossed athreshold value; a comparator adapted to compare the output voltage withthe reference signal; and a second selector to select a mode ofoperation of the charge pump based on the comparison.

Optionally, the charge pump controller, further comprises a multiplexerhaving a first channel coupled to the first selector and a secondchannel coupled to the comparator; the multiplexer being adapted toselect the first channel upon receipt of a signal from the slew ratedetector.

Optionally, the charge pump controller further comprises a sensorcoupled to the slew rate detector; the sensor being adapted to sense anamplitude of the source signal and to enable the slew rate detector uponidentifying that the amplitude is below a certain value.

The charge pump controller according to the second aspect of thedisclosure may comprise any of the features described above in relationto the method according to the first aspect of the disclosure.

According to a third aspect of the disclosure, there is provided anaudio system comprising a charge pump controller as defined in thesecond aspect of the disclosure, the charge pump controller beingcoupled to a charge pump and an amplifier coupled to the charge pump foramplifying the source signal; wherein the source signal is an audiosignal.

Optionally, the audio system comprises a processor for processing theaudio signal, wherein the first selector of the charge pump controlleris connected to the processor.

The audio system according to the third aspect of the disclosure mayshare features of the second aspect, as noted above and herein.

According to a fourth aspect of the disclosure, there is provided amethod of controlling a charge pump for converting an input voltage intoan output voltage with a conversion ratio, the charge pump beingoperable in a plurality of modes corresponding to different conversionratios, the method comprising the steps of detecting a rate of change ofan input signal function of the input voltage; generating a referencesignal upon identifying that a magnitude of the rate of change hascrossed a threshold value; comparing the output voltage with thereference signal; and selecting a mode of operation of the charge pumpbased on the comparison.

Optionally, the reference signal is varying with a rate of change thatis less than a rate of change of the input signal.

Optionally, the method comprises selecting the mode of operation toreduce the output voltage upon identifying that the output voltage isgreater than the reference signal and selecting the mode of operation toincrease the output voltage upon identifying that the output voltage isless than the reference signal.

Optionally, the plurality of modes comprises integer modes in which theconversion ratio is a fixed integer; and transition modes in which theconversion ratio varies between two integers.

Optionally, wherein the input signal is a multiple of the input voltage.

According to a fifth aspect of the disclosure, there is provided acharge pump controller for controlling a charge pump adapted to convertan input voltage into an output voltage with a conversion ratio, thecharge pump being operable in a plurality of modes corresponding todifferent conversion ratios, the charge pump controller comprising aslew rate detector adapted to detect a rate of change of an input signalfunction of the input voltage and to generate a reference signal uponidentifying that a magnitude of the rate of change has crossed athreshold value; a comparator adapted to compare the output voltage withthe reference signal; and a selector to select a mode of operation ofthe charge pump based on the comparison.

Optionally, the reference signal is varying with a rate of change thatis less than a rate of change of the input signal.

Optionally, the selector is adapted to select the mode of operation toreduce the output voltage upon identifying that the output voltage isgreater than the reference signal and to select the mode of operation toincrease the output voltage upon identifying that the output voltage isless than the reference signal.

Optionally, the selector comprises a state machine operable between aplurality of states, each state corresponding to a specific mode ofoperation of the charge pump.

Optionally, the selector further comprises a multiplexer coupled to thestate machine.

Optionally, the charge pump controller comprises a multiplier adapted togenerate the input signal as a multiple of the input voltage.

Optionally, the charge pump controller further comprises a sensorcoupled to the slew rate detector; the sensor being adapted to sense anamplitude of the source signal and to enable the slew rate detector uponidentifying that the amplitude is below a certain value.

The charge pump controller according to the fifth aspect of thedisclosure may comprise any of the features described above in relationto the method according to the fourth aspect of the disclosure.

According to a six aspect of the disclosure there is provided an audiosystem comprising a charge pump controller as defined according to thefifth aspect, the charge pump controller being coupled to a charge pump;and an amplifier for amplifying the source signal coupled to the chargepump; wherein the source signal is an audio signal.

The audio system according to the six aspect of the disclosure may sharefeatures of the fifth aspect, as noted above and herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is described in further detail below by way of exampleand with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of an audio circuit;

FIG. 2 is a time chart illustrating the working of the circuit of FIG.1;

FIG. 3 is another time chart illustrating the working of the circuit ofFIG. 1;

FIG. 4 is a flow chart of a method of operating a charge pump;

FIG. 5 is a diagram of an audio amplifier circuit implementing themethod of FIG. 4;

FIG. 6 is a time chart illustrating the working of the circuit of FIG.5;

FIG. 7 is a flow chart of another method of operating a charge pump;

FIG. 8 is a diagram of an audio amplifier circuit comprising a slewerfor implementing the method of FIG. 7;

FIG. 9 is a diagram showing a waveform proportional to a battery voltageand a reference signal generated by the slewer of FIG. 8;

FIG. 10 is a time chart illustrating the working of the circuit of FIG.8;

FIG. 11 is a charge pump state diagram.

DESCRIPTION

FIG. 1 illustrates a system 100 for amplifying an audio signal. Thesystem 100 includes an audio amplifier 110, a capacitive boost circuit120, a battery 140, a boost reference circuit 180, a pair of digitalprocessing units referred to as first unit 152, and second unit 154, anaudio digital to analog converter 160 and an output capacitor 170. Thecapacitive boost circuit 120 is used to provide a supply voltageVDD_speaker to the audio amplifier 110. The capacitive boost circuitincludes a ratio selection circuit 122 connected to a charge pump 124.The digital processing unit 152 has an input for receiving a digitalaudio input and an output connected to the boost reference circuit 180and to the second digital processing circuit 154. The output of thesecond digital processing unit 154 is connected to the audio amplifier110 via the audio DAC 160. The ratio selection circuit 122 has a firstinput for receiving a battery voltage and a second input for receiving avoltage reference provided by the boost reference unit 180. The chargepump 124 has an input connected to the battery 140 and an outputconnected to the audio amplifier 110. When the audio amplifier 110 is aclass-D amplifier, the system 100 may be referred to as a class-Hamplifier.

In operation, the digital audio input is processed by the digitalprocessing units 152 and 154 and converted to an analog signal which isthen provided to the audio amplifier 110. The audio amplifier 110 thenamplifies the audio signal which may be sent to a speaker for providingsound waves. The audio amplifier which in this case is a class Damplifier requires a VDD_Speaker voltage of for example 10 volts. TheVDD_Speaker voltage is provided by the charge pump 124 by boosting thebattery voltage. The boost reference circuit 180 is used to generate areference voltage Vref from the digital audio signal. When the digitalaudio signal increases, the voltage Vref increases as well.

FIG. 2 is a time chart illustrating the working of the system 100 ofFIG. 1. FIG. 2 shows the battery voltage 210, the output of the audioamplifier VAUDIO 220, the reference voltage 230, and the ratio selectionsignal, also referred to as Quantized Target signal QTZ 240.

The voltage reference Vref 230 is used as a target value that must beprovided by the charge pump. Stated another way, the voltage VDD_speakerprovided by the charge pump must be equal to, or greater than, thevoltage Vref. The ratio selection circuit 122 compares the batteryvoltage 210 with the reference voltage 230 and outputs the quantizedselection signal QTZ 240 to achieve VDD_speaker greater than or equal toVref. The reference voltage Vref increases with a non-infinite ratecalled the attack rate, and when the audio activity requires less power,a release time Tr is used before reducing the activity of the circuit.

Before time t1, the audio signal increases over a couple of periods. Theboost reference circuit 180 detects this increase and at time t1increases the reference voltage from 5V to 9.5 V. For example, the boostreference circuit 180 may monitor the amplitude of the audio signal andrecord the local maxima or crests values; and increase Vref accordingly.

At time t2, the amplitude of oscillation of the audio signal startsdecreasing, however the reference signal remains constant at 9.5V untiltime t3. The period between times t2 and t3 defines a so-called releasetime Tr during which the reference signal remains constant. Once thisrelease time has expired, the reference voltage Vref 230 decreases to5.5V.

Between the times t2 and t3, Vref 230 remains constant at 9.5V and thequantized signal 240 equals 3× the battery voltage, hence 12V, in orderto satisfy the condition:Vdd_Speaker=N*Battery voltage>Vref  (1)

in which N is an integer.

At time t4, the amplitude of oscillation of the audio signal decreasesfurther to about 3V. The voltage reference remains constant at 5.5V forthe release time Tr, until time t5.

Between the times t3 and t5, Vref remains constant at 5.5V and thequantized signal 240 equals 2× the battery voltage, hence 8V, in orderto satisfy the condition of equation (1).

At time t5, the reference voltage decreases to 3.5V and remains constantuntil time t6. Between the times t5 and t6 the quantized signal 240equals 1× the battery voltage, hence 4V, in order to satisfy thecondition of equation (1).

At time t6, the amplitude of oscillation of audio signal increases to9V. The voltage reference Vref increases to 9.5V and remains constant.Between the times t6 and t7 the quantized signal 240 is equal to 3× thebattery voltage, hence 12V, in order to satisfy the condition ofequation (1).

At time t7 the battery voltage 210 starts decreasing and as a result,the QTZ signal 240 also decreases. At time t8, the battery voltage is3.3V and the ratio selector switches from 3× to 4× the battery voltageto reach 13.2V (4×3.3) to maintain the condition of equation (1) as thevoltage battery keeps decreasing further.

In another scenario, the battery voltage may decrease, for example from4V to 3.5V with a fall time of 10 microseconds and then later rises backto 4V, again with a rise time of 10 microseconds. In this instance, thevariation in battery voltage from 4V to 3.5V would result in a variationof the VDD_speaker voltage, going from 12V to 10.5V, when the chargepump is operated with a 3× conversion ratio. Therefore, the variation inVDD_speaker would be three times larger than the variation of thebattery voltage. Consequently, the audio amplifier cannot reject thesharp and large ripples of the battery voltage, which becomes audible;hence affecting the quality of the audio signal provided by the speaker.

FIG. 3 presents another set of waveforms for illustrating the working ofFIG. 1, including the battery voltage 310, the audio signal 320, thereference voltage 330 and the VDD_speaker voltage 350.

At time t0, the reference voltage 330 increases from 5V to 9.5Vconsecutive to an increase in the amplitude of oscillation of the audiosignal.

At time t1, the quantized signal QTZ switches the conversion ratio from2× to 3× the battery voltage and the VDD_speaker 350 starts increasing.

As the power of the audio signal increases, more current is requiredfrom the battery 140. Since the battery 140 has an equivalent seriesresistance ESR, the battery voltage decreases. At time t1′ theVDD_speaker voltage 310 decreases due to the decrease in batteryvoltage.

At time t2, the battery voltage 310 reaches a level that triggers anincrease in the conversion ratio from 3× to 4× the battery voltage. As aresult, VDD_speaker 350 stops decreasing; however, at this point morecurrent is needed from the battery to ramp up the output capacitor 170and the battery voltage keeps decreasing further until time t3, at whichpoint no more current is used to ramp up Cout 170 and the batteryvoltage starts increasing.

At time t4 the battery voltage 310 is back to a level that issufficiently high to allow the conversion ratio to be reduced to 3× thebattery voltage. The battery voltage 310 keeps increasing.

At time t5 the charge pump resumes pumping, depleting the battery yetagain. When VDD_Speaker exceeds an arbitrary reference value, forexample 3Vbat-100 mV, then the charge pump remains inactive, (skippingstate) and Cout get discharged. Once VDD_Speaker drops below thereference value then pumping is restarted until a second reference forexample (3Vbat-50 mV). If VDD_Speaker keeps increasing the charge pumpreturns to the skipping state.

At time t6, the battery voltage reaches a level which triggers anincrease in the conversion ratio to 4× the battery voltage, hencestarting a new cycle of variation of the battery voltage.

As a result, the VDD_speaker voltage 350 displays an oscillatorybehaviour with large amplitude variations. Such oscillations are due tothe repetitive changes in the mode of operation of the charge pump, inthis example between 3× and 4× conversion ratios. Such oscillationsaffect the quality of the output signal.

FIG. 4 is a flow chart of a method of operating a charge pump forconverting an input voltage into an output voltage with a conversionratio. The charge pump is operable in a plurality of modes correspondingto different conversion ratios.

At step 410, a target value of the output voltage is identified. Forinstance, the target value may be a pre-set value. For example, thetarget value may be an operating voltage of a device powered by thecharge pump such as an audio amplifier. Alternatively, the target valuemay be changing over time, depending on the demand placed on the devicepowered by the charge pump. For instance, if the device is an amplifierfor amplifying a source signal, then the target value may depend on anamplitude of the source signal.

At step 420, the input voltage is sensed. For instance, the inputvoltage may be a battery voltage Vbat. At step 430, a product of theconversion ratio and the input voltage is calculated. At step 440, theproduct is compared with the target value. Then, a mode of operation ofthe charge pump may be selected based on the comparison.

If the product is less than the target value, then the conversion ratiomay be increased at step 450. If the product is equal to the targetvalue, then the conversion ratio may be either increased or maintainedas shown at step 460.

A plurality of products for different values of the conversion ratio maybe identified. Then the conversion ratio associated with the productthat is the closest to the target value may be selected. Differentproducts may be calculated for a same input voltage and different valuesof conversion ratios. Alternatively, pre-calculated product values maybe stored in a register. The product that is the closest to the targetvalue may be less than the target value, or equal to the target value orgreater than the target value.

In a first numerical example, if the target value is 10V, and the inputvoltage Vbat=3V; then the product of the conversion ratio N timesVbat:N.Vbat equals 6V for N=2, 9V for N=3, and 12V for N=4. In thisnumerical example, the product value the closest to the target value isthe product value of 9V corresponding to the conversion ratio N=3. Thisproduct value is less than the target value. So, at step 450 theconversion ratio may be increased from N=2 to N=3 if the initialconversion ratio was N=2; or maintained at N=3 if the initial conversionratio was N=3.

In a second numerical example, the target value is still 10V but theinput voltage Vbat=2.5V. In this scenario, the product N.Vbat equals 5Vfor N=2, 7.5V for N=3, 10V for N=4, and 12.5 for N=5. In this secondnumerical example, the product value the closest to the target value isthe product value of 10V corresponding to N=4. This product value isequal to the target value.

Depending on the application, the selection of the conversion ratio mayfurther require that the conversion ratio corresponds to a product thatis equal or greater than the target value. Referring back to the firstnumerical example, the first conversion ratio that corresponds to aproduct value equal or greater than the target value is N=4.

If the product is greater than the target value at step 445, theconversion ratio is maintained for a length of time also referred to asrelease time at step 470 and then the conversion ratio is decreased oncethe release time has expired at step 480. The release time may be apre-set length of time. For instance, the release time may be tens ofmilliseconds. The release time may be reset to zero after a change inconversion ratio; at which point a counter may count from zero up to adefined length of time.

Depending of the implementation, the input voltage, the target value andthe product may be digitised to provide digitised values.

FIG. 5 describes an audio amplifying system 500 according to thedisclosure. The system 500 includes an audio amplifier 510, a chargepump 520, a dynamic ratio selector 530, a battery 540, a set of digitalprocessing units 552, 554, an audio digital to analog converter 560 andan output capacitor 570.

The first digital processing unit 552 has an input for receiving adigital audio signal and an output coupled to the second digitalprocessing unit 554 and to the dynamic ratio selector 530. The audio DAC560 has an input coupled to the output of the second digital processingunit 554 and an output coupled to the audio amplifier 510. The audioamplifier 510 has a first input coupled to the output of the audio DAC560 and a second input coupled to the charge pump 520. The output of theaudio amplifier may be coupled to a speaker (not shown). The charge pump520 has a first input coupled to the dynamic ratio selector 530, asecond input coupled to the battery and an output coupled to both theoutput capacitor 570 and to the audio amplifier 510.

The dynamic ratio selector 530 has a ratio selection block 531, whichhas a first input coupled to the battery 540, a second input coupled tothe output of the first digital processing unit 1 552, and an outputcoupled to the charge pump 520. The ratio selector block 531 is directlyconnected to the output of the digital processing 552. A timer not shownmay be provided to indicate that a release time has expired. The timermay be implemented in different ways. For instance, the timer may be acounter coupled to the dynamic ratio selector 530. The dynamic ratioselector 530 may be implemented in different fashions. For instance, thedynamic ratio selector may be a state machine. A state machine isconfigured to define a new state based on the knowledge of the previousstate and on the input received. Such a state machine may be implementedas a processor configured to run an algorithm or as a binary searchcircuit.

The digital processing units 552, 554 may be merged into a single unit.In this case the output signal dig_audio2 would be provided to the ratioselector 530.

It will be appreciated that an analog version of the circuit of FIG. 5may be implemented. In this case the components 552, 554 and 560 may bereplaced by an analog filter. However, a smaller circuit size can beachieved when implementing the circuit 500 digitally.

FIG. 6 is a time chart illustrating the working of the circuit of FIG.5. FIG. 6 shows the battery voltage 610, the audio signal Vaudio 620provided by the audio amplifier 510 and the VDD speaker voltage 650generated by the charge pump 520. The input audio signal Dig_audio 1, isnot represented. The input audio signal is a non-amplified version ofVaudio shifted by a delay, for example −50 μs with respect to the signalVaudio.

In operation, the digital processing units 552 and 554 may be used toincrease the sampling rate of the signal also referred to as upsampling;and also, to perform signal interpolation to create additional datasignal. The digital processing units may also be used to filter thedigital signal. The second processing unit 554 may introduce a delay.For instance, the second digital processing 554 may introduce a delaytime of 50 μs. This means that the output of the audio DAC is known inadvance, hence allowing ramping up the voltage VDD_Speaker if required.

The dynamic ratio selector 530 receives a signal Dig_audiol from thedigital processing unit 552, that is a processed version of the inputaudio signal, and a voltage Vbat from the battery 540. The dynamic ratioselector 530 identifies a mode of operation of the charge pumpcorresponding to a specific conversion ratio Ni in which Ni is aninteger (1, 2, 3, 4 . . . ) at time ti, then calculates a productPi=Ni.Bi in which Bi is a value of the battery voltage obtained at timeti. Depending on the implementation, the battery voltage may bedigitized. The dynamic ratio selector 530 then compares the product Piwith a target value and adjusts the conversion ratio accordingly. Inthis example, the target value Ai is an estimated value of the signalVaudio at time ti. The target value may be estimated based on theamplitude of the signal Dig_audio1, and a known gain of the amplifier510. For instance, the dynamic ratio selector 530 may calculate a ratioof Pi/Ai to identify the integer Ni such that the productN_(i)·B_(i)≥A_(i).

The battery voltage Vbat and the input audio signal may be sensedcontinuously, so that the required conversion ratio N can be calculatedand if necessary updated in time. The amplitude of the input signal maybe measured at a point in time corresponding to a local maximum, alsoreferred to as crest value. As a result, the voltage VDD_speaker (equalsto N.Vbat) is maintained at a sufficient level for operating theamplifier.

At time t1, the dynamic ratio selector 530 receives a value B1 from thebattery voltage and calculates a product P1=N1*B1. The product P1 isless than a threshold value A1 and the ratio selector 530 increases theconversion ratio from 2× to 3×. For instance, A1 may have an amplitudeof 8V measured at a maximum of the input audio signal sinusoidaloscillation. In this case if VBAT=B1=3.8V, then a conversion of N1=3 isrequired.

Between the times t1 and t2 VDD_Speaker 650 increases while the batteryvoltage decreases. At time t2, the product P2=3B2 is less than athreshold value A2 and the ratio selector 530 increases the conversionratio from 3× to 4×.

The dynamic ratio selector 530 may send a reset signal to a timer, forinstance a counter, counter to reset the release time. The counter thenstart counting for a certain length of time, for example 100 ms, andthen provides a release signal indicating that to the dynamic ratioselector that the release time has expired. The release time thereforeallows the conversion ratio N to remain at its value, in this case 4,for a certain amount of time even if the input signal decreases. TheVDD_speaker voltage 650 stops decreasing; however, at this point morecurrent is needed from the battery to ramp up the output capacitor 570and the battery voltage 610 keeps decreasing until time t3 at whichpoint no more current is used to ramp up Cout 570 and the batteryvoltage 610 stop decreasing.

At time t3, the product is above the threshold value A3 and the ratioselector maintains a 4× ratio.

At time t4, the battery has returned to an intermediate value, however,the product P4 is such that the ratio is maintained to 4×. As a result,the voltage battery level remains constant until time t5.

At time t5, the amplitude of oscillation of the audio signal decreases,resulting in an increase in Vbat and VDD_Speaker. The product is morethan the threshold value A5, however the ratio is maintained constant at4× for a release time duration Tr. At time t6 the conversion ratio isreduced to 3×.

By updating the conversion ratio based on the product N.Vbat and bymaintaining the conversion ratio N for a release time Tr uponidentifying a decrease of the input signal, the dynamic ratio selectorprevents the VDD_speaker voltage from oscillating.

FIG. 7 is another flow chart of a method of operating a charge pump forconverting an input voltage into an output voltage with a conversionratio. The charge pump is operable in a plurality of modes correspondingto different conversion ratios.

At step 710, a rate of change of an input signal function of the inputvoltage is detected. For instance, the input signal may be a multiple ofthe input voltage. At step 720, a reference signal is generated uponidentifying that a magnitude of the rate of change has crossed athreshold value. For instance, if the rate of change corresponds to adecrease in the input voltage, then a reference signal with a negativeslope may be generated. Similarly, if the rate of change corresponds toan increase in the input voltage, then a reference signal with apositive slope may be generated. The reference signal may be implementedin different ways with different waveform shapes. In an exemplaryembodiment, the reference signal may be a linear function or a staircasefunction with a periodic increase or a periodic decrease.

At step 730, the output voltage is compared with the reference signal.At step 740 a mode of operation of the charge pump is selected based onthe comparison. For instance, if the output voltage is greater than thereference signal, then a mode of operation of the charge pump may beselected to reduce the output voltage. On the contrary if the outputvoltage is less than the reference signal, then a mode of operation ofthe charge pump may be selected to increase the output voltage.

FIG. 8 shows a diagram of an audio circuit 800 for amplifying an audiosignal. FIG. 8 shares many similar components to those illustrated inFIG. 5. The same reference numerals have been used to representcorresponding components and their description will not be repeated forthe sake of brevity. The audio circuit 800 includes a charge pumpcontroller 850 for implementing the method according to FIG. 7. Thecharge pump controller 850 includes a first controller 530 for providinga first control signal; and a second controller 810 for providing asecond control signal.

In this example, the first controller is provided by the dynamic ratioselector 530 as described with reference to FIG. 5. However, it will beappreciated that the first controller may be implemented by another typeof ratio selector, for instance ratio selector 122 described withreference to FIG. 1. Optionally, the charge-pump controller 850 may alsoinclude a load detector 830 for enabling the second controller 810. Theload detector 830 may be implemented as a comparator for comparing anamplitude of the input signal with a minimum threshold value. If thesignal amplitude is less than the threshold value, the load may bereferred to as light. The input signal may be a periodic signal havinglocal maxima (crests) and local minima (valleys) and the amplitude ofthe input signal may be measured at a crest of the input signal.

The circuit 810 includes a multiplier 812, a slew rate detector alsoreferred to as slewer 814, a comparator 816, a mode selector such as astate machine 818, and a multiplexer 819.

The slewer 814 may be implemented digitally. For instance, if thebattery voltage Vbat is coded in 8 bits, a battery voltage Vbat of 3.8Vwould translate into a digital number Dbat=194. If N=3, then the NBATwould translate into a digital number of 582. If the battery voltagevaries with a rate of change that exceed a certain threshold, then theslewer 814 changes the slope of the input NVbat to reduce it.

The multiplier 812 has a first input coupled to the dynamic ratioselector 530 and a second input coupled to the battery 540. The slewer814 has an input coupled to the output of the multiplier 812, and twooutputs: a first output coupled to the multiplexer 819 and a secondoutput coupled to the comparator 816. Optionally the slewer 814 may alsobe provided with a second input for receiving an output of the dynamiclight load detector 830. The comparator 816 has a first input, forinstance, an inverting input coupled to the output of the charge pump520 and a second input, for instance, a non-inverting input coupled tothe second output of the slewer 814.

The multiplexer 819 has a first input coupled to the output of thecomparator 816, a second input coupled to the dynamic ratio selector 530and a third input coupled to the first output of the slewer 814. Theoutput of the multiplexer is coupled to the final state machine FSM 818.

The FSM 818 has an input for receiving an output of the multiplexer 819and an output coupled to the charge pump 520. The FSM 818 may also beprovided with another input for receiving a signal indicating whetherthe battery voltage has increased or decreased. This signal may be usedby the FSM to increment or decrement the conversion ratio selection uponreceipt of the signal provided by the multiplexer.

In operation, the multiplier 812 calculates a product of a ratio Nprovided by the ratio selector 530 times the battery voltage Vbat; andoutputs a signal referred to as N.BAT that varies with time.

The slewer 814 may be enabled by default, for instance upon starting thecircuit 800. Alternatively, the slewer 814 may be enabled by the dynamiclight load detector 830. For instance, the dynamic light load detector830 may detects that the circuit 800 is operating in a light loadcondition and send a logic signal to the slewer to enable it. A lightload condition may be identified if an amplitude of the Dig_audio1signal is below a certain threshold value. Once enabled, the slewer 814provides two signals, a logic signal also referred to as slewing signalto the multiplexer 819; and a reference signal, also referred to asslope signal, to the comparator.

The slewing signal allows selecting a specific channel of themultiplexer 819. For instance, when the slewing signal is a logic high,for example a logic 1, the multiplexer may enable the channel coupled tothe comparator 816. Alternatively, when the slewing signal is low, forexample a logic 0, the multiplexer 819 may enable the channel coupled tothe dynamic ratio selector 530.

If the slewing signal is low, the FSM 818 receives the selection signalQTZ from the dynamic ratio selector 530. For instance, if QTZcorresponds to a ratio N=3, the FSM will select RATIO3X.

If the slewing signal is high, the FSM 818 receives the regulationsignal REG from the comparator 816. In this instance, the FSM identifiesthe value of the REG signal. For example, if REG=0, then the FSM doesnot select another mode; however, if REG=1, indicating that VDD_SPKR isnot high enough compared to ramp R.BAT, then the FSM 818 select adifferent mode to ensure that VDD_SPKR eventually tracks R.BAT.Following on the previous example the FSM 818 may change from a RATIO3Xto a RATIO4X.

The slewer 814 receives the signal N.BAT from the multiplier 812 andidentifies a rate of variation d(N.BAT)/dt of the signal N.BAT overtime. If the rate of variation is greater than a threshold value, theslewer 814 generates the reference signal R.BAT associated with a slowerrate of variation. The reference signal is time-dependent and may bedefined by the product of a number R and the battery voltage Vbat; inwhich R is a real number that increases or decreases with time. In anexemplary embodiment the threshold value of the rate of variation may beset to 1V/ms. In this instance, the reference signal may be generated ifd(N.BAT)/dt exceeds 1V/ms.

FIG. 9 illustrates the output signal N.BAT 910 provided by themultiplier 812, and the reference signal R.BAT 920 provided by theslewer 814. In this example, the waveform 910 varies from a first levelhaving a value of 2Vbat at time t0, to a second level having a value3Vbat at time t1. To simulate a slower variation, the slewer 814generates the reference signal 920. In this example the slope signal isprovided by a staircase function having a periodic increase. The value Rincreases at a given frequency defined by a time interval T, from R=2 toR=3. The waveform 920 reaches the second level at a later time t2>t1.

The value of R increases incrementally by a fixed incremental value atevery time interval T. In the present example the incremental value isset to 0.1. The incremental value may be a pre-set value. By changingthe incremental value and/or the time interval T, the slope of the slopesignal can be adjusted and therefore the duration of the slope signal.The incremental value and the time internal define a step which may beprogrammable.

Referring back to FIG. 8, the comparator 816 then compares the referencesignal with VDD_speaker provided by the charge pump 520. The comparator816 then provides a regulation signal REG to the FSM 818 via themultiplexer 819. This regulation signal may be a logic signal, such as alogic high or logic low to select a mode stored in the FSM 818.

The final state machine 818 may be configured to store a plurality ofmodes for operating the charge pump 520. These modes may include bothinteger modes corresponding to N times the battery voltage in which N isan integer as well as transition modes for passing between integermodes. Upon receipt of the regulation signal, the FSM 818 selects aparticular mode and outputs a control signal to the charge pump 520 foroperating the charge pump with a specific conversion ratio.

FIG. 10 is a time chart illustrating the working of the circuit of FIG.8. The chart includes the regulation signal 1010 provided by thecomparator 816, a conversion ratio state 1020 of the control signal alsoreferred to as operating ratio signal provided by the FSM 818, theslewing signal 1030 provided by the slewer 814, the battery voltage1040, the audio signal Vaudio 1050, the output voltage VDD speaker 1060provided by the charge pump, the output signal 1070 of the multiplier812; and the reference signals 1082, 1084 generated by the slewer 814.

The circuit 800 can operate between two modes: a first mode alsoreferred to as normal mode, in which the FSM 818 receives a ratioselection signal QTZ from a ratio selector such as the dynamic ratioselector 530 and a second mode, referred to as slew mode, in which theFSM 818 receives the regulation signal REG from the comparator 816.

Initially, before time t1 the circuit 800 operates in the first mode.The battery voltage 1040 and the output voltage VDD_speaker 1060 remainsstable. The value of the VDD_speaker voltage 1060 remains constant aslong as the battery voltage 1040 remains constant. The FSM 818 520receives a selection signal QTZ from the dynamic ratio selector 530. TheFSM 818 generates a control signal based on the selection signal QTZ tooperate the charge pump. The selection signal QTZ may be a digitalnumber, for example if the digital battery voltage is 194, then QTZ is582 then the FSM generates a control signal for example MODE3X foroperating the charge pump with a conversion ratio of 3× the batteryvoltage.

At time t1, the battery voltage 1040 decreases; as a result, the outputsignal 1070 of the multiplier and the voltage VDD_speaker 1060 droprapidly. The slewer 814 identifies that the variation of the signal 1070(N*dVbat/dt) is equal or greater than a certain threshold value. Theslewer sends the slewing signal 1030 to the multiplexer 819. Themultiplexer 819 now selects a first channel connected to the output ofthe comparator 816 and the circuit 800 operates in the second mode. Theslewer 814 generates a first slope signal 1082 having a negative slope,and the comparator 816 then compares VDD_speaker 1060 with the slopesignal 1082. At time t1, VDD_speaker 1060 is lower than the slope signal1082; as a result, the comparator 816 increases the regulator signal1010 from a logic low to a logic high value. The FSM 818 receives theregulator signal 1010 via the multiplexer 819 and increases the ratiofrom 3× the battery voltage to 4× the battery voltage. As a consequence,VDD_speaker 1060 increases.

At the time t2, VDD_speaker 1060 is greater than the slope signal 1082and the regulation signal 1010 decreases from a logic high to a logiclow. The control signal or operating ratio signal 1020 now operates thecharge pump with a 3× ratio.

Between the times t2 and t6, VDD_speaker 1060 oscillates around thefirst slope signal 1082. Eventually at time t6, the slewing signal 1030goes low and the control signal 1020 is maintained with a 3× conversionratio.

Between the times t6 and t7, the circuit 800 operates in the first mode.At time t7, the battery voltage 1040 increases back to its former value.This variation in battery voltage is detected by the slewer 814 whichincreases the slewing signal 1030 from a logic low to a logic high. Themultiplexer 819 then selects the first channel connected to the outputof the comparator 816 and the circuit 800 operates in the second mode.The slewer 814 generates a second slope signal 1084 having a positiveslope, and the comparator 816 then compares VDD_speaker 1060 with thesecond slope signal 1084. At time t7, VDD_speaker 1060 is greater thanthe slope signal 1084; as a result, the comparator 816 increases theregulator signal 1010 from a logic low to a logic high value. The FSM818 receives the regulator signal 1010 via the multiplexer 819 anddecreases the ratio from 3× the battery voltage to 2× the batteryvoltage. As a consequence, VDD_speaker 1060 decreases.

Between the times t7 and t12, VDD_speaker 1060 oscillates around thesecond slope signal 1084. Eventually at time t13, the slewing signal1030 goes low and the quantized signal 1020 is maintained with aconversion ratio of 3× the battery voltage. The circuit returns to thefirst mode of operation.

FIG. 11 illustrates a charge pump state diagram. As mentioned above, thefinal state machine 818 described with reference to FIG. 8, may storeseveral modes for operating the charge pump. It will be appreciated thatthe FSM 814 may store many different modes and is not limited to therepresentation illustrated in FIG. 11.

The diagram of FIG. 11 shows six different modes, including two integermodes 1110 and 1120 and four transition modes, 1112, 1114, 1122 and1124. For instance, the mode 2× 1110 corresponds to a mode in which thecharge pump generates an output voltage that is twice the input voltage;in other words, the conversion rate is 2. Similarly, the mode 3× 1120corresponds to a mode in which the charge pump generates an outputvoltage of three times the input voltage, that is to say the conversionrate is 3.

The transition between the mode 2× 1110 and the mode 3× 1120 is notspontaneous and requires a transition period during which the chargepump is operating in a transition mode. For instance, when passing fromthe mode 2× 1110 to the mode 3× 1120, the charge pump operates for acertain time in the transition mode 2×3 1112.

It will be appreciated that the charge pump 520 of the circuit of FIGS.5 and 8 may be implemented in different fashions. For instance, thecharge pump 520 may include a plurality of switches and capacitorsactivated with a plurality of drive signals. An adaptive table may beprovided to generate the drive signals based on the control signalgenerated by the FSM.

A skilled person will appreciate that variations of the disclosedarrangements are possible without departing from the disclosure. Forinstance, it will be appreciated that the circuits of FIGS. 5 and 8 maybe implemented in analog or digital forms. Accordingly, the abovedescription of the specific embodiment is made by way of example onlyand not for the purposes of limitation. It will be clear to the skilledperson that minor modifications may be made without significant changesto the operation described.

What is claimed is:
 1. A method of controlling a charge pump forconverting an input voltage into an output voltage with a conversionratio, the charge pump being operable in a plurality of modescorresponding to different conversion ratios, the method comprising thesteps of: identifying a target value of the output voltage; calculatinga product of the conversion ratio and the input voltage; comparing theproduct with the target value; and selecting a mode of operation of thecharge pump by increasing or decreasing the conversion ratio based onthe comparison; and maintaining the conversion ratio associated with theselected mode of operation for a length of time before decreasing theconversion ratio.
 2. The method as claimed in claim 1, wherein theconversion ratio is an integer.
 3. The method as claimed in claim 2,further comprising the steps of: identifying a plurality of products fordifferent values of the conversion ratio, and selecting a conversionratio associated with a product that is the closest to the target value.4. The method as claimed in claim 1, wherein the target value varieswith time and wherein identifying the target value comprises sensing asource signal to estimate the target value.
 5. The method as claimed inclaim 4, further comprising the step of: recording amplitude maxima ofthe source signal.
 6. The method as claimed in claim 1, furthercomprising the step of: calculating a ratio of the product over thetarget value to select the mode of operation.
 7. The method as claimedin claim 1, the method further comprising the steps of: detecting a rateof change of an input signal function of the input voltage; and uponidentifying that a magnitude of the rate of change has crossed athreshold value, generating a reference signal, comparing the outputvoltage with the reference signal; and selecting a mode of operation ofthe charge pump based on the comparison.
 8. The method as claimed inclaim 7, wherein the reference signal is varying with a rate of changethat is less than a rate of change of the input signal.
 9. A charge pumpcontroller for controlling a charge pump adapted to convert an inputvoltage into an output voltage with a conversion ratio, the charge pumpbeing operable in a plurality of modes corresponding to differentconversion ratios, the controller comprising a first selector forselecting a mode of operation of the charge pump; the first selectorcomprising a first input for coupling to a voltage supply providing theinput voltage; and a second input for coupling to a source signal; thefirst selector being configured to identify a target value of the outputvoltage; calculate a product of the conversion ratio and the inputvoltage; compare the product with the target value; and select a mode ofoperation of the charge pump by increasing or decreasing the conversionratio based on the comparison, and maintaining the conversion ratioassociated with the selected mode of operation for a length of timebefore decreasing the conversion ratio.
 10. The charge pump controlleras claimed in claim 9 wherein the conversion ratio is an integer; andwherein the first selector is configured to identify a plurality ofproducts for different values of the conversion ratio, and to select aconversion ratio associated with a product that is the closest to thetarget value.
 11. The charge pump controller as claimed in claim 9,wherein the first selector is configured to calculate the target valuebased on the source signal.
 12. The charge pump controller as claimed inclaim 11, wherein the first selector is configured to record amplitudemaxima of the source signal.
 13. The charge pump controller as claimedin claim 9 further comprising a slew rate detector adapted to detect arate of change of an input signal function of the input voltage and togenerate a reference signal upon identifying that a magnitude of therate of change has crossed a threshold value; a comparator adapted tocompare the output voltage with the reference signal; and a secondselector to select a mode of operation of the charge pump based on thecomparison.
 14. The charge pump controller as claimed in claim 13,further comprising a multiplexer having a first channel coupled to thefirst selector and a second channel couple to the comparator; themultiplexer being adapted to select the first channel upon receipt of asignal from the slew rate detector.
 15. The charge pump controller asclaimed in claim 13, further comprising a sensor coupled to the slewrate detector; the sensor being adapted to sense an amplitude of thesource signal and to enable the slew rate detector upon identifying thatthe amplitude is below a certain value.
 16. An audio system comprising acharge pump controller as claimed in claim 9 coupled to a charge pumpand an amplifier coupled to the charge pump for amplifying the sourcesignal; wherein the source signal is an audio signal.
 17. The audiosystem as claimed in claim 16 comprising a processor for processing theaudio signal, wherein the first selector of the charge pump controlleris connected to the processor.